1. Field of the Invention
The present invention relates to a control apparatus for a direct injection engine, which comprises an injector for directly injecting fuel into a combustion chamber, and comprises a lean NOx catalyst in the exhaust path of the engine.
2. Description of the Related Art
One conventional injection engine comprises an injector for directly injecting fuel into a combustion chamber, and improves fuel economy by lean burn by means of stratified combustion. In an engine of this type, a lean NOx catalyst that adsorbs NOx in an excess oxygen atmosphere and releases NOx as the oxygen concentration decreases is inserted in the exhaust path, and can purify NOx even in lean combustion.
When the engine comprises the lean NOx catalyst, it must be regenerated or refreshed by releasing NOx when its adsorbed NOx amount increases. For example, when lean combustion continues for a long period of time, the catalyst is regenerated by periodically changing the air-fuel ratio to a value equal to or lower than a stoichiometric air-fuel ratio for a predetermined period of time. Upon catalyst regeneration, it is required to cause easy NOx desorption reaction and to reduce desorbed NOx so as not to directly exhaust it. For this purpose, a deoxidizing agent such as CO must be sufficiently present in the atmosphere.
As a technique for regenerating such lean NOx catalyst, for example, a technique that makes additional injection during an expansion stroke in addition to main injection for stratified combustion upon catalyst regeneration is known, as disclosed in Japanese Patent Laid-Open No. 10-274085.
In a conventional apparatus described in that reference, CO in exhaust gas increases as a result of additional injection during the expansion stroke, and desorption of NOx from the lean NOx catalyst and deoxidization of that NOx are promoted. However, since additional injection is done during the expansion process in addition to main injection of fuel in an amount corresponding to the required torque of an engine, and fuel injected by this additional injection is exclusively used to produce CO for desorption and deoxidization of NOx from the lean NOx catalyst, fuel economy readily suffers.
In addition to this catalyst regeneration technique that desorbs adsorbed NOx to increase the NOx amount adsorbed by the catalyst, the present applicant has also proposed a technique for supplying CO upon regenerating the NOx catalyst polluted by SOx in exhaust by heating it to high temperatures so as to maintain performance of the NOx catalyst for a long period of time.
The present invention has been made in consideration of the above situation, and has as its object to provide a control apparatus for a direct injection engine, which can efficiently regenerate a lean NOx catalyst that adsorbs NOx, and can improve the regeneration effect while suppressing a drop in fuel economy.
In order to achieve the above object, the first invention is a control apparatus for a direct injection engine, which comprises an injector for directly injecting fuel into a combustion chamber, performs lean combustion by setting an air-fuel ratio higher than a stoichiometric air-fuel ratio in a low-load range of the engine, and comprises a lean NOx catalyst that adsorbs NOx in an excess oxygen atmosphere and releases NOx as an oxygen concentration decreases, comprising: air-fuel ratio changing means for changing the air-fuel ratio between an air-fuel ratio higher than the stoichiometric air-fuel ratio to an air-fuel ratio substantially equal to or less than the stoichiometric air-fuel ratio; exhaust gas recirculation means for recirculating some exhaust gas to an intake system; and control means for, when the air-fuel ratio is changed from the air-fuel ratio higher than the stoichiometric air-fuel ratio to the air-fuel ratio substantially equal to or less than the stoichiometric air-fuel ratio, controlling to divisionally execute fuel injection from the injector in at least two injections including leading injection which starts within an intake stroke period, and trailing injection which starts within a compression stroke period, and controlling the exhaust gas recirculation means to recirculate the exhaust gas so as to implement catalyst regeneration control.
In the first invention, the trailing injection in the catalyst regeneration control preferably starts within a duration of a middle period of the compression stroke (second invention).
According to the apparatus of this invention, the catalyst regeneration control is done when the air-fuel ratio has changed from a lean air-fuel ratio to the rich side, and during the catalyst regeneration control, an air-fuel ratio substantially equal to or lower than a stoichiometric air-fuel ratio is set, and divided injection is done in intake and compression strokes. For this reason, the CO amount in exhaust increases while assuring combustion stability, and NOx emission from the engine decreases by exhaust gas recirculation. Hence, the CO amount becomes relatively larger than the NOx amount as the sum of NOx released from the lean NOx catalyst upon catalyst regeneration and NOx in exhaust gas, thus promoting NOx desorption and deoxidization.
Also, in the first or second invention, the leading injection in the catalyst regeneration control preferably starts within a former half of the intake stroke (third invention). With this control, fuel injected by leading injection is sufficiently dispersed, and its vaporization and atomization are promoted, thus improving combustion stability.
Also, an injection amount of the leading injection in the catalyst regeneration control can be set to be not less than xc2xc a total injection amount (fourth invention). In this way, fuel injected by leading injection effectively contributes to combustion.
Especially, injection amounts of the leading and trailing injections in the catalyst regeneration control are preferably set to be substantially equal to each other (fifth invention). With this invention, the combustion stability effect of leading injection and the CO increase effect of trailing injection can be satisfactorily obtained. In a very-low load range with a small fuel supply amount, since the divided injection amounts approach a minimum controllable injection amount (minimum pulse width), if different amounts of fuel are injected by leading and trailing injections, the smaller injection amount may fall below the minimum injection amount. However, if equal injection amounts are set, such situation can be avoided.
In one of the first to fifth inventions, when the air-fuel ratio changing means is constructed to set the air-fuel ratio to be substantially equal to or less than the stoichiometric air-fuel ratio in an operating range on a higher-load side than an operating range in which lean combustion is made, and to change the air-fuel ratio in correspondence with a change in running state, the catalyst regeneration control can be done when the operating range shifts from the operating range in which lean combustion is made to the operating range in which the air-fuel ratio is set to be substantially equal to or less than the stoichiometric air-fuel ratio (sixth invention).
With this invention, when the operating range shifts from that for lean combustion to that at substantially the stoichiometric air-fuel ratio or less as a result of a change in accelerator opening, the catalyst is effectively regenerated.
In this case, when the operating range shifts from the operating range in which lean combustion is made to the operating range in which the air-fuel ratio is set to be substantially equal to or less than the stoichiometric air-fuel ratio, intake/compression divided injection including leading information which starts within the intake stroke period and trailing injection which starts within the compression stroke period may be done for a predetermined period of time, and fuel injection may then be divisionally done within the intake stroke period (seventh invention).
With this invention, when catalyst regeneration has progressed to a satisfactory level by intake/compression divided injection, and the CO amount required for desorption and deoxidization of NOx is decreased to some extent, divided injection during the intake stroke period is started, thus improving fuel economy and the like.
In one of the first to seventh inventions, when an NOx adsorption amount of the lean NOx catalyst becomes not less than a predetermined value during lean combustion, the air-fuel ratio may be changed to be equal to or less than the stoichiometric air-fuel ratio, and regeneration control may be done (eighth invention).
With this invention, even when lean combustion continues for a long period of time, the catalyst can be regenerated effectively.
In this case, when the NOx adsorption amount of the lean NOx catalyst becomes not less than a predetermined value during lean combustion, the air-fuel ratio may be changed to be equal to or less than the stoichiometric air-fuel ratio, intake/compression divided injection including leading injection which starts within the intake stroke period and trailing injection which starts within the compression stroke period may be done for a first setting time, combined injection or divided injection may be done within the intake stroke period for a second setting time, and the engine may then return to lean combustion (ninth invention).
With this invention, when catalyst regeneration has progressed to a satisfactory level by intake/compression divided injection, and the CO amount required for desorption and deoxidization of NOx is decreased to some extent, combined injection or divided injection during the intake stroke period is started, thus improving fuel economy and the like.
The 10th invention is a control apparatus for a direct injection engine, which comprises an injector for directly injecting fuel into a combustion chamber, performs lean combustion by setting an air-fuel ratio higher than a stoichiometric air-fuel ratio in a low-load range of the engine, and comprises a lean NOx catalyst that adsorbs NOx in an excess oxygen atmosphere and releases NOx as an oxygen concentration decreases, comprising: air-fuel ratio changing means for changing the air-fuel ratio between an air-fuel ratio higher than the stoichiometric air-fuel ratio to an air-fuel ratio substantially equal to or less than the stoichiometric air-fuel ratio; and control means for, when the air-fuel ratio is changed from the air-fuel ratio higher than the stoichiometric air-fuel ratio to the air-fuel ratio substantially equal to or less than the stoichiometric air-fuel ratio, controlling to divisionally execute fuel injection from the injector in at least two injections including leading injection which starts within an intake stroke period, and trailing injection which starts within a compression stroke period, and controlling to retard the ignition timing of the engine from the ignition timing of MBT (controlling the ignition timing of the engine to be later than the ignition timing of MBT). MBT is the omission of Minimum advance for Best Torque.
In the apparatus of this invention, the trailing injection in the catalyst regeneration control preferably starts within a duration of a middle period of the compression stroke (11th invention).
According to the apparatus of this invention, the catalyst regeneration control is done when the air-fuel ratio has changed from a lean air-fuel ratio to the rich side, and during the catalyst regeneration control, an air-fuel ratio substantially equal to or lower than a stoichiometric air-fuel ratio is set, and divided injection is done in intake and compression strokes. For this reason, the CO amount in exhaust increases while assuring combustion stability, and NOx emission from the engine decreases since the ignition timing is retarded. Hence, the CO amount becomes relatively larger than the NOx amount as the sum of NOx released from the lean NOx catalyst upon catalyst regeneration and NOx in exhaust gas, thus promoting desorption and deoxidization of NOx.
Accordingly, the present invention increases the amount of CO for promoting the regeneration of the lean NOx catalyst by setting the trailing injection timing within the compression stroke in which the ignition timing is short and the vaporization and atomization of fuel is deteriorated relatively. Further the present invention improves combustion stability during a recirculation of exhaust gas by setting the injection start timing of the trailing injection within a middle period of the equally divided three period of compression stroke where piston speed is higher so as to urge a mixture of air and fuel of the trailing injection in the compression stroke on The trailing injection timing set within the compression stroke.
Further, the lean NOx catalyst is refreshed (hereinafter referred to as xe2x80x9cregeneratedxe2x80x9d) by releasing and reducing its absorbed NOx from the lean NOx catalyst so as to increase allowable absorbing amount of NOx.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.